Recent years have seen tremendous growth in the number and variety of display devices available to the public. Computers (whether desktop, laptop, or notebook), personal digital assistants (PDAs), mobile phones, and thin LCD TVs are but a few examples. Although some of these devices can use ordinary ambient light to view the display, most include a backlight to make the display visible.
Many such backlights fall into the categories of “edge-lit” or “direct-lit”. These categories differ in the placement of the light sources relative to the output face of the backlight, which output face defines the viewable area of the display device. In edge-lit backlights, a light source is disposed along an outer border of the backlight construction, outside the area or zone corresponding to the output face. The light source typically emits light into a light guide, which has length and width dimensions on the order of the output face and from which light is extracted to illuminate the output face. In direct-lit backlights, an array of light sources is disposed directly behind the output face, and a diffuser is placed in front of the light sources to provide a more uniform light output. Some direct-lit backlights also incorporate an edge-mounted light, and are thus capable of both direct-lit and edge-lit operation.
It is known to use an array of cold cathode fluorescent lamps (CCFLs) as the light sources for direct-lit backlights. It is also known for the diffuser to be in the form of a stiff sheet or plate that is mounted in a frame at a fixed position from the light sources. The mechanical stiffness of the plate helps keep the diffuser at a nominally fixed position relative to the light sources across the surface of the output face under normal handling conditions, using only edge mounting around the periphery of the diffuser. The diffuser also functions as a stable substrate against which additional light management films can be placed. Such additional films are often thin and flexible relative to the stiff diffuser plate, and can include in some cases an additional thin diffusing film, a prismatic brightness enhancement film such as Vikuiti™ brand Brightness Enhancement Film (BEF) available from 3M Company, and a reflective polarizing film such as Vikuiti™ brand Dual Brightness Enhancement Film (DBEF) available from 3M Company.
Thin film multilayer reflective polarizers can be made by an extrusion process in which a multitude of alternating light-transmissive polymer materials are coextruded through a die, optionally passed through one or more layer multipliers, then cast onto a casting wheel or surface, and subsequently stretched to form a refractive index mismatch Δnx between adjacent layers along an in-plane x-direction and a refractive index match (Δny=0) along an orthogonal in-plane y-direction. See, for example, U.S. Pat. Nos. 5,486,949 (Schrenk et al.); 5,882,774 (Jonza et al.); 6,531,230 (Weber et al.); and 6,827,886 (Neavin et al.). Such all-polymer thin film multilayer reflective polarizers do not require a separate substrate for formation or handling, but are often sold in the form of thin flexible films or sheets.
In general, the physical size of optical components such as diffuser plates and polarizers used in liquid crystal displays should substantially match the size of the output screen. As the demand for larger screen sizes grows, so too does the requirement for physically larger samples of such optical components, including reflective polarizers. However, most reflective polarizers used in displays are thin film multilayer devices that rely on coherent constructive or destructive interference of light from neighboring layer interfaces. Such constructive or destructive interference is a strong function of the individual layer thicknesses, as well as other geometric factors. Great care is typically needed to ensure that the layers are controlled to within a narrow tolerance to ensure proper operation of the reflective polarizer. Even greater care is necessary as the physical size of these polarizers increases. Increasing the physical size of thin film reflective polarizers also magnifies potential mechanical problems such as wrinkling, warping, and delamination.